Jointly owned Property IHT discount

Does anyone have any experience of changing property ownership into joint names to take better advantage of the valuaiton discounts mentioned in the HMRC manual? Client’s value is £1.5m (mainly in property). The son has suggested purchasing a fraction of each of his mother’s properites to get them into joint names now to reduce future IHT. This would be at full market value, but as it would be a very small percentage share, no CGT or SDLT would be triggered. I wonder how this would be viewed by HMRC and if there is a minium percentage share that HMRC would expect the ‘smaller’ owner to have, ideally say 1%? (or could it be even less than 1% - eg 0.5% ?), Thanks for any thoughts on this.

A very interesting ‘planning’ query. If it was put another way, namely that "a property is jointly owned between H and W and S, with S only owning 1%, does a joint ownership discount apply? My immediate thought would be that a valuation discount would apply. The concern here is that it seens to be artificially arranged, do you then get caught under GAAR or some other anit-avoidance terms? It may well be that the even on a ‘open market sale’, a loss to to the estate principle will apply, particularly if the said S is the ultimate beneficiary.